The invention relates to a combined process for the preparation of polycarbonate from bisphenols and diaryl carbonates, the monophenol liberated being reacted with phosgene again for the preparation of the diaryl carbonate, and the alkali metal halide forming in the preparation of the diaryl carbonate being converted by electrochemical oxidation into chlorine, the chlorine being recycled to the preparation of the phosgene. The alkali solution likewise formed can be used again for the preparation of the diaryl carbonate.

The preparation of aromatic polycarbonates by the melt transesterification process is known and is described, for example, in “Schnell”, Chemistry and Physics of Polycarbonates, Polymer Reviews, Vol. 9, Interscience Publishers, New York, London, Sydney 1964, in D. C. Prevorsek, B. T. Debona and Y. Kersten, Corporate Research Center, Allied Chemical Corporation, Moristown, N.J. 07960, “Synthesis of Poly(ester)carbonate Copolymers” in Journal of Polymer Science, Polymer Chemistry Edition, Vol. 19, 75-90 (1980), in D. Freitag, U. Grigo, P. R. Müller, N. Nouvertne, BAYER AG, “Polycarbonates” in Encyclopedia of Polymer Science and Engineering, Vol. 11, Second Edition, 1988, pages 648-718 and finally in Dres. U. Grigo, K. Kircher and P. R. Müller “Polycarbonate” in Becker/Braun, Kunststoff-Handbuch [Plastics Handbook], Volume 3/1, Polycarbonate, Polyacetale, Polyester, Celluloseester [Polycarbonates, Polyacetals, Polyesters, Cellulose esters], Carl Hanser Verlag, Munich, Vienna 1992, pages 117-299.
WO 93/13085 A1 describes a process for the preparation of polycarbonate, in which the monophenol which was liberated in the transesterification process can be reacted with a carbonyl dihalide (e.g. phosgene) to give the diaryl carbonate again. In this process, the carbonyl dihalide is prepared from CO and a metal halide compound. This method of preparation via a catalyst is complicated and uneconomical.
The preparation of the diaryl carbonate used in the melt transesterification process for aromatic polycarbonates, for example by the phase boundary process, is described in principle in the literature, cf. for example in Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964), pages 50/51.
U.S. Pat. No. 4,016,190 A describes a process for the preparation of diaryl carbonates which is operated at temperatures of >65° C. In this process, the pH is initially set low (pH 8 to 9) and then high (10 to 11).
Optimizations of the process by improving the mixing and maintaining a narrow temperature and pH profile as well as isolation of the product are described in EP 1 219 589 A1, EP 1 216 981 A2, EP 1 216 982 A2 and EP 784 048 A1.
In these known processes, however, a high residual phenol value in the waste water of these processes, which may pollute the environment and present the waste water treatment plants with an increased waste water problem, necessitates complicated purification operations. Thus, WO 0307639 A1 describes a removal of the organic impurities in the waste water by an extraction with methylene chloride.
According to known processes, the sodium chloride-containing solution is freed from solvents and organic residues and then disposed of.
According to WO 2000/078682 A1 or U.S. Pat. No. 6,340,736 A, the sodium chloride-containing waste waters can be purified by ozonolysis and then used in the sodium chloride electrolysis. A disadvantage of this process is the very expensive ozonolysis.
EP 1 894 914 A2 describes the recycling of sodium chloride-containing waste water from the diaryl carbonate preparation by phase boundary phosgenation of sodium phenolate (DPC) through a chloralkali electrolysis with an increase in the sodium chloride concentration by separation of the reaction waste water from the wash phases and increased water transport by a novel membrane technology. However, reuse of monophenols, alkali solution and halide formed in the process has not been described.
Starting from the prior art described above, it is the object to provide a polycarbonate preparation process which gives products in high purity and good yield and permits reduction of the environmental pollution or waste water problem in the waste water treatment plants by maximized recycling of byproducts and process waste water solutions which originate from the polycarbonate production.
Furthermore, in the recycling, the conversion of sodium chloride into chlorine and sodium hydroxide solution and optionally hydrogen should be effected with minimum use of energy and therefore the protection of resources.